Known thermal printing ink media of this type include those shown in FIGS. 7 and 8, as described in GAZO DENSI GAKKAISHI, Vol. 11, No. 1, pp. 3-9 (1982) and ibid, Vol. 16, No. 2, pp. 84-88 (1987), respectively.
In FIG. 7, ink medium 100 is a composite plastic film composed of 15 to 25 .mu.m thick layer 101 for current passage, 5 to 20 .mu.m thick semiconductive layer 102, and 400 to 500 .ANG. thick conductive layer 103. With a constant voltage (either AC or DC) applied to recording head 104, an electric current passes as indicated by the dotted line to induce a breakdown in the vicinity of the boundary between semiconductive layer 102 and conductive layer 103. Conductive layer 103 and a part of semiconductive layer 102 are thus transferred to transfer material 105 and simultaneously fixed thereon by the electrically generated heat to provide a semipermanent record.
In FIGS. 8-(a) and (b), ink medium 110 has a three-layered structure composed of base layer 111 for current passage, conductive layer 112, and ink layer 113 or a four-layered structure composed of base layer 111 for current passage, conductive layer 112, release layer 114, and ink layer 113. Ink medium 110 is characterized by providing a separate conductive layer independent on an ink layer. Electricity is applied from recording head 115 to conductive layer 112 via base layer 111 to cause base layer 111 to generate heat thereby to transfer the ink of ink layer 113 to transfer material 116.
Further, JP-A-63-191684 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") proposes a process for producing a thermal printing ink medium shown in FIG. 9. In FIG. 9, thermal printing ink medium 120 comprises heat fusible ink layer 121, conductive layer 122, and resistive layer 123. Resistive layer 123 is composed of high resistive layer 123a which has an electrical resistance to electrically generate heat sufficient for melting the heat-fusible ink and low resistive layer 123b which has a lower electrical resistance than high resistive layer 123a, with low resistive layer 123b being on the side to be in contact with a recording electrode 124. The process proposed is characterized in that a coating composition containing a resin and a conductive material is spray-coated on high resistive layer 123a to a thickness of from 0.1 to 3 .mu.m to form low resistive layer 123b.
The above-mentioned conventional techniques have their own disadvantages. In the case of ink medium 100, since the transferred image layer consists of conductive layer 103 and semiconductive layer 102, conductive layer 103 must have conductivity and a visual color. In order to render layer 103 conductive, carbon particles, etc. should be added thereto. This not only narrows choice of materials but makes coloring difficult. Further, since both layers 101 and 102 have an insufficient thickness for serving as a support for film formation, ink medium 100 cannot be produced without difficulty in laminating of thin films and without an increase in the cost of production. Furthermore, since image signals return through conductive layer 103, which also serves as an ink layer, printing unavoidably results in a structural change to lose conductive layer 103, which is an essential constituent of the ink medium. This causes instable printing quality. Besides, it is difficult to regenerate the lost part of conductive layer 103, making it difficult to repeatedly use the ink medium.
In the case of ink medium 110, since base layer 111, which plays the main role in heat generation, is remote from ink layer 113, the heat generated in base layer 111 is dissipated till it reaches ink layer 113, resulting in a low energy efficiency. Further, since electricity from recording head 115 is passed directly to base layer 111, the main part for heat generation, the surface of base layer 111 has large heat generation energy due to the contact resistance with recording head 115, resulting in damage of the surface of base layer 111. Furthermore, since conductive layer 112 contacts with ink layer 113 either directly or via release layer 114, it is liable to be damaged. As a result, the printing tends to be instable. Besides, regeneration of the damaged conductive layer 112 is difficult, making it difficult to repeatedly use the ink medium.
In the case of ink medium 120, the contact resistance between the medium and recording head 125 may be reduced by providing low resistive layer 123b. However, since low resistive layer 123b is very thin, and, as a result, high resistive layer 123b is necessarily fairly thick, the heat generated in high resistive layer 123a is dissipated while passing therethrough till it reaches ink layer 121. Therefore, an improvement in energy efficiency cannot be expected. It is necessary to set the peak temperature of heat generation in high resistive layer 123a at a high value before the reduction in energy efficiency accompanied by heat dissipation in high resistive layer 123a can be compensated for, but this gives rise to another problem of an increase in requisite energy. Further, the diffusion of the heat in high resistive layer 123a results in undesired widening of the area of ink layer 121 to be melted and transferred, i.e., a reduction in resolving power. Furthermore, when ink layer 121 is transferred, conductive layer 122 in direct contact therewith is liable to be damaged, and stable printing is hardly obtained. It is difficult to regenerate the damaged part of conductive layer 122, making it difficult to repeatedly use the ink medium.